Interference cancellation system

ABSTRACT

An interference cancellation system has a combiner which removes an undesired component from a main signal including both a desired signal and an undesired signal received by a main antenna. A power sensor measures output power of a main signal. A weight control circuit is coupled with an output of the power sensor. An auxiliary antenna receives an undesired signal. A variable complex weight control circuit adjusts an undesired signal from the auxiliary antenna according to the output of the weight control circuit to provide an input signal to the combiner. The variable complex weight control circuit operates so that the output power becomes a minimum.

FIELD OF THE INVENTION

The present invention relates to an interference cancellation systemwhich cancels undesired interference waves in the direction of sidelobes of a receiver antenna.

BACKGROUND OF THE INVENTION

An interference cancellation system cancels undesired waves which arereceived by a high gain antenna positioned in the direction of a desiredsignal, by combining the desired signal with an undesired signalreceived by another antenna positioned in the direction of the undesiredwave, the same amplitude and opposite phase.

FIG. 5 shows a prior interference cancellation system. In FIG. 5, anoutput signal of a main antenna (1) (which is called a main signal) isconverted to an intermediate frequency signal by a local oscillator (5)and a mixer or a combiner (3). An output signal of an auxiliary antenna(2) which is called a reference signal is, after being converted to anintermediate frequency signal by a local oscillator (5) and a mixer (4),combined with a weighting signal w which is the output of a weightcontrol circuit (8) in a variable complex weight circuit (7), so that apresumed signal of an undesired signal included in a main signal isprovided. The numeral (6) is a combiner which functions to subtract thepresumed undesired signal from the main signal. The output of thecombiner (6) is the output of the interference cancellation system. Thereference signal is multiplied with the output signal of theinterference cancellation system in the multiplier (10). The numeral (9)is a low-pass filter, and the combination of the multiplier (10) and thelow-pass filter (9) provides a correlation circuit. The output of thecorrelation circuit is applied to the variable complex weight circuit(7) through the weight control circuit (8) so that a correlation loop isprovided. The correlation loop senses a residual component of anundesired signal included in the output signal.

DRAWBACKS OF A PRIOR ART

In the above structure, when both the desired signal and undesiredsignal are wideband signals, a mutual spectrum density function isspread over a wide frequency range. Further, since those signals arerandom signals, the average of a correlation value is not constant, butdeviates. Therefore, a prior correlation circuit with a multiplier (10)and a low-pass filter (9) is not enough for providing an accuratecorrelation value, and an undesired signal is not sufficientlycancelled. This fact is the problem in a prior interference cancellationsystem.

SUBJECT OF THE INVENTION

An object of the present invention is to overcome the disadvantage ofthe prior art.

It is also an object of the present invention to provide an excellentinterference cancellation system irrespective of the bandwidth and/ormodulation system of a signal, by using the power of an output signal.

The important feature of the present invention is the use of a feedbackloop which adjusts phase and amplitude of a reference signal so that thesignal power contained in the desired signal band (which is calledinband power) becomes minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first embodiment of the present invention,

FIG. 2 is a modification of the first embodiment,

FIG. 3 is a second embodiment of the present invention,

FIG. 4 is a third embodiment of the present invention, and

FIG. 5 is a block diagram of a prior interference cancellation system.

PREFERRED EMBODIMENTS OF THE INVENTION

The theoretical explanation of the present invention is summarized asfollows. Assuming that d is a desired signal component, and i is anundesired signal component, and n is a noise component, a main signal xis expressed as follows.

    x=d+i+n                                                    (1)

A reference signal y has undesired signal component i_(a) and noisecomponent n_(a). When the ratio i_(a) /n_(a) in an auxiliary antenna issufficiently large, y is expressed as follows.

    y=i.sub.a +n.sub.a ≃i.sub.a                  (2)

The following relation is satisfied between i and i_(a).

    i.sub.a =k·exp(j·θ)·i     (3)

where k is a gain ratio of two antennas in the direction of an undesiredsignal, and is the phase difference which relates to the difference inthe propagation path length. An output signal z of the interferencecancellation system is expressed as follows by using the above symbols.

    z=d+(1-w·K)·i+n                          (4)

where K=k·exp(j·θ) (5)

If no correlation exists among d, i and n, the power of the outputsignal z is expressed as follows.

    E[|z|.sup.2 ]=E[|d|.sup.2 ]+E[|1-w·K|.sup.2 ·|i|.sup.2 ]+E[|n|.sup.2 ](6)

where E[|z|² ] is the average power of the output signal z. Therefore,it should be noted that an undesired signal is sufficiently cancelled bydetermining a weight signal w so that the power of an output signal zbecomes minimum. Equation (6) shows that the power of the output signalz has the minimum value. To determine the weight signal w, an ordinarymethod of minimizing a function which has two real variables isapplicable. Therefore, it is easy to determine the weight signal w.

FIG. 1 shows the first embodiment which uses the above theory of thepresent invention. In FIG. 1, an output signal y (which is a referencesignal) of an auxiliary antenna (2) is multiplied with a weight signal wwhich is the output of a weight control circuit (8), to provide apresumed undesired signal in an output signal x (main signal) of a mainantenna (1). The numeral (13) is a combiner which functions to subtractthe presumed undesired signal from the main signal The output of thecombiner (13) is converted to an intermediate frequency signal by alocal oscillator (5) and a mixer (3). The numeral (11) is a bandpassfilter which passes proper bandwidth relating to that of the desiredsignal. The numeral (15) is an intermediate frequency amplifier. Theoutput of the intermediate frequency amplifier (15) is the output z ofthe present interference cancellation system. The numeral (12) is apower sensor which measures inband power of the output signal z. Anoutput information of the power sensor (12) is applied to the weightcontrol circuit (8). The weight control circuit (8) determines theweight signal w so that the inband power in the output signal z becomesminimum, and the weight signal w is applied to the variable complexweight circuit (7). The weight signal w is obtained by minimizing thefunction which has two real variables as mentioned before. Therefore,the weight control circuit is implemented simply by using a microprocessor. An embodiment operation of the weight control circuit is asfollows.

(Initial Operation)

This operation determines the initial values for the optimum controloperation which is described later. The initial operation carries outthe following operations.

(a) The initial value w₀, the variable range of the amplitude (δ), andthe variable range of the phase (φ) of the weight signal w arearbitrarily given.

(b) The output signal power is measured when the value of the weightsignal w is w₀.

(c) The amplitude and the phase of the weight signal are shifted by (δ),and (φ), respectively, from the initial value w₀, and the output signalpower for each shifted values are measured (4 shifted values, or 8shifted values are tested).

(d) The output signal powers measured in the items (b) and (c) arecompared with one another, and the weight signal w₁ which provides theminimum signal power is obtained.

(e) When the value w₁ which is obtained in the item (d) differs from w₀,the value w₀ is substituted by the value w₁, and the operations afterthe item (b) are repeated.

When the value w₁ is equal to w₀, then, the values (δ) and (φ) arehalved.

(f) When the values (δ) and (φ) reach lower than the predeterminedvalues, then, the initial operation finishes, and the next optimumcontrol operation begins. Otherwise, the operation after the item (b) isrepeated.

(Optimum Control Operation)

The optimum control operation is carried out so that the weight signal wfollows the optimum value, and includes the following operations.

(g) The weight signal w is set to the final value w₀ which is obtainedin the above initial operation. The values (δ) and (φ) are set to asmall value, so that the output signal is not affected significantly bythose values.

(h) The output signal power is measured, when the weight signal is w₀.

(i) The output signal power is measured, when the phase of the weightsignal is shifted by ±0 from w₀.

(j) The output signal powers measured in items (h) and (i) are comparedwith each other, and the weight signal which provides the minimum outputpower is obtained. The value which provides the minimum output power isregarded as a new final value w₀.

(k) The weight signal w is set to w₀, and the output signal power forthe revised weight signal is measured.

(l) The output signal power is measured when the amplitude of the weightsignal is shifted by (+δ) from w₀.

(m) The output signal powers obtained in the items (k) and (l) arecompared with each other, and the weight signal which provides theminimum output signal is obtained. That value is regarded as a new finalvalue w₀.

(n) The items after (h) are repeated.

It should be appreciated that the above operation is only one example,and any method for obtaining the minimum value of a two variablefunction can be used in the present invention. For instance, aNewton-Lafson method, a method of steepest descent, a conjugate gradientmethod, or a random search method can be used in the present invention.

In the embodiment of FIG. 1, when the amplitude and the phase of thereference signal are controlled with the errors less than 0.2 dB, and1.8° degrees, respectively, through the necessary bandwidth, theundesired signal is suppressed by more than 32 dB.

As understood in equation (6), the weight signal w does not depend uponthe power level of the desired signal, the undesired signal, and thenoise. Therefore, the present invention is useful even when S/N (theratio of the signal power to the noise power), or D/U (the ratio of thedesired signal power to the undesired signal power) is small. Forinstance, when S/N=10 dB, D/U=-10 dB, and k=10, the present inventioncan suppress the undesired signal by more than 30 dB.

FIG. 1 shows the embodiment which measures the power at the intermediatefrequency stage. As an alternative, the power may be measured at thebaseband stage.

FIG. 2 shows the modification of the embodiment of FIG. 1. The featuresof FIG. 2 are the use of a down converter (14) instead of a localoscillator (5) and a mixer (3), and the deletion of an intermediatefrequency amplifier (15). The circuit of FIG. 2 has the similar effectto that of FIG. 1.

FIG. 3 shows another embodiment of the present invention. The embodimentof FIG. 3 is useful when there are a plurality of desired signals like amulti-carrier communication system which includes FDMA (FrequencyDivision Multiple Access), and all of those desired signals areinterfered by undesired signals.

In FIG. 3, the output signal y of the auxiliary antenna (2) ismultiplied to the output signal w of the weight control circuit (8) sothat the product which is the presumed undesired signal included in theoutput signal x of the main antenna (1) is obtained. The numeral (13) isa combiner which functions to subtract the presumed undesired signalfrom the main signal. The output of the combiner (13) is converted tothe intermediate frequency signal by a frequency converter (14) which isimplemented by a down-converter. The numerals (11-1) through (11-N) arebandpass filters, each of which passes a signal of each bandwidth ofeach desired signal. The numerals (12-1) through (12-N) are sensorswhich measure signal power in each bandwidth of each desired signal. Theoutputs of the sensors (12-1) through (12-N) are applied to a weightcontrol circuit (8), which determines the weight signal w so that outputpowers of each of the sensors become minimum.

Since the embodiment of FIG. 3 handles only the signal included in eachbandwidth of each desired signal, it is useful to compress the undesiredsignal in a multi-carriers communication system.

An example of the operation of the weight control circuit is as follows.

(a) The average received power level d_(on) (n=1 . . . N) of eachdesired signal is calculated. That calculation is generally carried outin the link design stage.

(b) The output power z_(on) (n=1 through N) of each bandpass filter isobtained. Then, the difference from d_(on) is calculated as follows.

    P.sub.n =E[|Z.sub.on |.sup.2 ]-d.sub.on (n=1-N) (7)

(c) The value n₀ which gives the maximum difference (P_(n)) is obtained,and the value w is determined so that (P_(no)) becomes minimum. Theabove control is essentially a step track of two variables. Othernon-linear optimum methods are available in the present invention.

(d) The steps (b) and (c) are repeated.

FIG. 4 shows a block diagram of still another embodiment of the presentinvention. The feature of FIG. 4 is the improvement of the embodiment ofFIG. 3 so that a plurality of undesired signals are cancelled. In FIG.4, the numerals (2-1) through (2-M) are auxiliary antennas, each ofwhich receives an undesired signal in a different direction from eachother. The output signal y_(m) (m=1 through M) is multiplied with w_(m)(m=1 through M), which is output from the weight control circuits (8),and the product is the presumed undesired signal for cancelling theundesired signal in the output signal x of the main antenna (1). Thenumeral (13) is a combiner which functions to subtract the presumedundesired signal from the main signal. The output of the combiner (13)is frequency-converted by the down-converter (14) so that anintermediate frequency signal is obtained. The numerals (11-1) through(11-N) are bandpass filters for passing desired signals in each desiredband. The numerals (12-1) through (12-N) are power sensors, each ofwhich measures output power in each bandwidth of each desired signal.The outputs of the power sensors (12-1) through (12-N) are applied tothe weight control circuit (8), which determines the weight signal w_(m)(m=1 through M) so that the inband power in each bandwidth of eachdesired signal becomes a minimum.

One example of the operation of the weight control circuit (8) is asfollows.

(a) The average receive power level d_(on) (n=1 through N) of eachdesired signal is calculated. That power level is generally calculatedat the link design stage.

(b) The output power z_(on) (n=1 through N) of each bandpass filter isobtained, and the following value is calculated.

    P.sub.n =[E{|Z.sub.on |.sup.2 }-d.sub.on ]/d.sub.on n=(1-n)                                                   (8)

(c) The following object function is introduced.

    Q=ΣP.sub.n                                           (9).

The value {w_(m) } (m=1 through M) is determined so that the objectfunction Q becomes for a set of {W_(m) } of complex weight signals.

(d) The steps (b) and (c) are repeated.

Although simple control operations are shown for the embodiments ofFIGS. 3 and 4, other conventional optimum methods for multi-objectfunctions can be used in the present invention.

EFFECT OF THE INVENTION

The effects of the present invention are enumerated as follows.

(1) The control is accurate, since the inband power of an output signalis used, instead of a prior correlation value. Therefore, an undesiredsignal is suppressed sufficiently due to a small error signal, and thecircuit structure is simple.

(2) Even when both the desired signal and the undesired signal arewideband signals, and the signal to noise ratio (S/N) of the mainantenna is small, excellent suppression of undesired signal is obtained,since the inband output power can be measured accurately even in theabove cases.

(3) The excellent suppression effect of an undesired signal is obtainedeven when the undesired signal level is higher than the desired signallevel.

(4) The excellent compression effect of an undesired signal is obtainedeven for a multi-carrier communication system like FDMA, by measuringonly the signal power included in the bandwidth of each of the desiredsignals.

(5) Even when there are a plurality of undesired signals, the presentinvention is applicable.

APPLICATION TO INDUSTRY

The present invention can compress an undesired signal in a widebandcommunication system, and the structure of the circuit is simple.Therefore, the present invention is useful in particular for thatcommunication system.

What is claimed is:
 1. An interference cancellation system whichreceives a main signal, including a desired signal and an undesiredsignal in a direction of a sidelobe of a main antenna, and a referencesignal, which has essentially only the undesired signal, saidinterference cancellation system suppressing an undesired signalcomponent in the main signal to provide a main signal which has only adesired signal component, said interference cancellation systemcomprising:a main antenna positioned in a direction of the desiredsignal; an auxiliary antenna positioned in a direction of the undesiredsignal; a subtractor coupled with an output of said main antenna; anoutput terminal coupled with an output of said subtractor; powermeasuring means, coupled with the output of said subtractor, formeasuring a power of the output of said subtractor in a high frequencyband; a variable complex weight circuit, coupled with said auxiliaryantenna and said subtractor, said variable complex weight circuitadjusting the undesired signal received by the auxiliary antenna; andcontrol means, coupled to said variable complex weight circuit and saidpower measuring means, for controlling said variable complex weightcircuit according to said output power.
 2. An interference cancellationsystem according to claim 1, wherein at least two power measuring meansare provided, each having a bandwidth and a center frequency relating toeach desired signal, said variable complex weight control circuit beingcontrolled so that outputs of each of the power measuring means becomesa minimum.
 3. An interference cancellation system according to claim 1,wherein a plurality of variable complex weight control circuits and aplurality of auxiliary antennas are provided for each of a plurality ofundesired signals received from various directions.